a) Field of the Invention
The present invention relates to control apparatus and method for a motor to prevent switching devices of a motor drive circuit from being overloaded applicable to an electric vehicle during a motor halt state or during an extremely low speed running of the motor.
b) Description of the Related Art
In an electric vehicle in which an AC power is supplied from a motor drive circuit to a running motor to drive the motor to run the electric vehicle, an alternating current flows through each semiconductor switching device of the motor drive circuit during a time of a normal revolution of the motor.
However, a DC current flows in a particular switching device during a halt state of the motor or during an extremely low revolution speed of the motor so that a heat loss in the corresponding particular switching device is abruptly increased.
When the motor is revolved at a normal speed, the approximately same current flow occurs through the switching devices so that an even heat loss is distributed over the whole switching devices. In this case, a casing temperature and device junction temperature indicate low values respectively. In addition, a difference in the casing temperature between each switching device is small. However, in a case where the motor is locked so as to be in a halt state or in an extremely low speed running state, the DC current flows through the particular switching device so that an abrupt rise in the casing temperature of the particular switching device occurs. In addition, the difference in each casing temperature is accordingly increased.
In order to prevent each of the junction temperatures in the respective switching devices of the motor drive circuit from exceeding an allowable limit of the junction temperature, a first previously proposed apparatus for preventing an overload on the motor drive circuit has been proposed in which the casing temperature for each switching device is detected, and, when the casing temperature of each of the switching devices exceeds a threshold value thereof, and an output current of the motor drive circuit to the motor is reduced.
The first previously proposed overload preventing apparatus is exemplified by a Japanese Patent Application Fist Publication No. Heisei 5-115106 published on May 7, 1993.
A second previously proposed overload preventing apparatus in which a locked state of the motor is detected and, when the motor becomes the locked state, the output current of the motor drive circuit is limited.
The second previously proposed overload preventing apparatus is exemplified by a Japanese Patent Application First Publication No. Heisei 8-19150 published on Jul. 23, 1996.